The use of MFD generally predates the adoption of the SI prefix mu (μ) symbol, often simplified with a u, to mean "micro", or "1 millionth" or "1/1000000". This adoption happened somewhere around 1960. MFD can be found especially on schematics from before then, and always means "microfarad".
Nowadays, we use the following SI prefixes most often:
p (pico, or 1/1,000,000,000,000), n (nano, or 1/1,000,000,000), μ (micro, or 1/1000,000), m (milli, or 1/1000), k (kilo, or 1000), M (mega, or 1,000,000), G (giga, or 1,000,000,000), and T (tera, or 1,000,000,000,000).
There are many more besides these (including centi for 1/100, deci for 1/10, deca for 10, and so on and so forth) but in the electronics field you will more often find the ones listed above.
If you don't take context into account, it may be easy to confuse μ/micro, m/milli, and M/mega.